This mod increases the number of mod slots that each weapon/tool has, by 1 per level. Tier 1 = 1 Mod Slot, Tier 6 = 6 Mod Slots. Also, check out his Trader Tier Reward System.
The forum topic of the mod is here.
Credits: EpicSpire
Quality Mod Slot Boost
Probably worth adding to the description that this does not apply to existing items, only to the ones found after applying the modlet.
Keep this mod active, very good, congratulations